BST Based Continuously Tunable Delay Line for Full-Duplex Applications

This paper presents a continuously tunable delay line based on coupled transmission-line resonators whose resonant frequencies and coupling coefficients are controlled using BST varactors. By independently tuning these parameters, the proposed architecture enables fully analog adjustment of both the operating center frequency and the achievable group delay, offering great flexibility. The reflection group delay produced by the two-resonator network is transformed into a transmission group delay using a circulator. The prototype operates with tunable center frequency over 3.4–3.7 GHz, covering one of the sub-6-GHz 5G frequency bands. This prototype achieves a continuously adjustable group delay from 1.5 ns to 7.5 ns while maintaining a flat response over a 20 MHz bandwidth. The measured results highlight the suitability of the proposed approach for applications such as self-interference cancellation, where large tuning range and fine resolution of group delay are simultaneously required.